An organic luminous element is hopeful for a display device of the next generation, because the organic luminous element is a self-luminous element, and therefore, for example, the device does not require a back light required in a liquid crystal display device, and has a wide viewing angle.
A sectional view of an element structure of a general organic luminous element is shown in FIG. 2. The organic luminous element has a structure in which an organic layer 22 is sandwiched by a cathode 21 and an anode 23. When a DC power supply 24 is connected to this organic luminous element, holes and electrons are injected into the organic layer 22 from the anode 23 and the cathode 21, respectively. The injected holes and electrons move to the counter electrodes in the organic layer 22 due to an electric field formed by the power supply 24. The electrons and the holes are combined again within the organic layer 22 in the course of the movement to generate excitons. Luminescence is observed in a process in which energy of the excitons is deactivated. Luminescent colors are different depending upon energy inherent in the excitons, and light having a wavelength of energy substantially corresponding to a value of an energy band gap inherent in the organic layer 22 is generated.
In order to take out the light generated in the organic layer to the outside, a material, which is transparent in a visible light region, is used for at least one of the electrodes. A material, which has a low work function, is used for the cathode in order to facilitate injection of electrons into the organic layer. For example, a material such as aluminum, magnesium, or calcium is used. A material such as an alloy of these metals or aluminum-lithium alloy may be used for durability and a lower work function.
On the other hand, a material having a large ionization potential is used for the anode owing to its easiness to inject holes. In addition, since the cathode does not have transparency, a transparent material is often used for this electrode. Therefore, in general, an ITO (Indium Tin Oxide), gold, indium zinc oxide (IZO), or the like is used.
In recent years, in an organic luminous element using a low molecular material, in order to increase luminous efficiency, as shown in FIG. 3(a), the organic layer 22 may be constituted by plural layers (in this case, four layers). An electron injection layer 31 is provided in order to make it easy to inject electrons into the organic layer from the cathode 21. Similarly, a hole injection layer 34 is used for improving easiness to inject holes from the anode 23. These injection layers are often formed at thickness of about 5% to 20% with respect to thickness of the organic layer 22. This makes it possible to increase the number of carriers for both electrons and holes to be injected into the organic layer. As materials used for these layers, a material having a value of electron affinity close to the work function of the cathode 21 is used in the case of the electron injection layer 31, and a material having a value of ionization potential close to the value of the anode 23 is used in the case of the hole injection layer 34.
On the other hand, materials used for an electron transport layer 32 and a hole transport layer 33 are materials having high mobility of carriers to be transported. In addition, a material having high fluorescence is used for one of the layers, and the layer contributes to luminescence. In other words, one of the layers also functions as a luminous layer. At present, since there are many luminous materials having an electron transport property, in general, the electron transport layer 32 also functions as the luminous layer. In this layer, the carriers for both electrons and holes, which have been injected and moved, are combined again, and light is emitted to the outside. Thus, a material of emitting light having a desired wavelength is used for the electron transport layer 32. As representative materials, there are an aluminum-quinoline complex for green, a europium complex for red, and the like. Note that, in the case in which the electron transport layer 32 and the hole transport layer 33 are used as the luminous layer, one of the layers is not always constituted by one material but may be constituted by a material obtained by scattering a fluorescent pigment (guest material) in a certain material (host material).
Luminance of the organic luminous element formed in this way is proportional to a current as shown in FIG. 4(a) and is in a nonlinear relation with respect to a voltage as shown in FIG. 4(b). Therefore, in order to perform gradation control, it is better to control the organic luminous element according to a value of current.
An example in the case in which organic luminous elements are incorporated in a passive matrix type display device is shown in FIG. 5. Organic luminous elements 55 are arranged in points of intersection of segment signal lines 56 and common signal lines 57 such that a current from the segment signal lines 56 is flown to any one of the organic luminous elements on the identical segment signal lines 56 according to an operation of a common driver 52. Gradation display is performed by the current flowing to the segment signal lines 56.
Therefore, a segment driver 51 is required to be a driver of a current output type.
On the other hand, in the case of an active matrix type, display devices are roughly divided into those of two systems, namely, a voltage drive system and a current drive system.
The voltage drive system is a method of using a source driver of a voltage output type, converting a voltage into a current inside a pixel, and supplying the converted current to organic luminous elements.
The current drive system is a method of using a source driver of a current output type, giving only a function of retaining a value of current, which is outputted for one horizontal scanning period, inside a pixel, and supplying the same value of current as the source driver to organic luminous elements.
An example of a circuit structure inside a pixel of the voltage drive system is shown in FIG. 6. A voltage supplied from a source signal line 60 is applied to a driving transistor 62 through a transistor 66 within a period of selecting the pixel. Note that a capacitor 65 is used for retaining information during one frame even after the period of selecting the pixel has ended.
A current flows from an EL power supply line 64 to an organic luminous element 63 according to a gate voltage of the driving transistor 62—drain current characteristic. It is possible to change an amount of current flowing to the organic luminous element 63 by changing a value of voltage to be applied to the source signal line 60.
However, in this system, there is a problem in that, if there is fluctuation in a voltage/current characteristic of the driving transistor 62, according to the fluctuation, fluctuation is caused in the current flowing to the organic luminous element.
This pixel circuit is often formed of a low-temperature polysilicon process. In the low-temperature polysilicon process, unevenness is easily caused in an amount of laser irradiation used at the time of polycrystallization, and fluctuation is also caused in characteristics of a transistor according to this unevenness of irradiation. In such a voltage drive system, there is a problem in that, since it is difficult to eliminate this unevenness of irradiation in a process, streaks corresponding to a direction of laser irradiation are caused, and unevenness of display is caused.
On the other hand, examples of the current drive system are shown in FIGS. 7 and 8. The system of FIG. 7 uses a current copier system in a pixel circuit. The system of FIG. 8 uses a current mirror system.
A circuit at the time of operation of a pixel 74 in FIG. 7 is shown in FIGS. 9(a) and (b).
When a pixel is selected, as shown in FIG. 9(a), a signal is outputted from a gate driver 71 such that a gate signal line 61a of a row of the pixel brings a switch into a conduction state and a gate signal line 61b of the line brings a switch into a non-conduction state. A state of the pixel circuit at this point is shown in FIG. 9(a). At this point, a current flowing to the source signal line 60, which is a current attracted into a source driver 17, flows through a path indicated by dotted line 101. Thus, a current identical with the current flowing to the source signal line 60 flows to a transistor 72. Then, a potential of a node 102 changes to a potential corresponding to a current/voltage characteristic of the transistor 72.
Next, when the pixel changes to an unselected state, the circuit is changed to a circuit as shown in FIG. 9(b) by the gate signal lines 61. A current flows from the EL power supply line 64 to the organic luminous element 63 through a path of dotted line indicated by 103. This current depends upon the potential of the node 102 and the current/voltage characteristic of the transistor 72.
In FIGS. 9(a) and (b), the potential of the node 102 does not change. Therefore, a drain current flowing to the identical transistor 72 is identical in FIGS. 9(a) and (b) Consequently, a current of the same value as the value of current flowing to the source signal line 60 flows to the organic luminous element 63. Even if there is fluctuation in the current/voltage characteristic of the transistor 72, values of currents 101 and 103 are not affected in principle, and uniform display without influence of fluctuation in characteristics of a transistor can be realized.
Similarly, in the case of the current mirror system of FIG. 8, a current flowing to the source signal line 60 through a path indicated by Iw in FIG. 10(a) flows in the pixel at the time when a row is selected. A voltage corresponding to a gate potential at the time when the current Iw flows to a transistor 82b is applied to a node 105.
A current through a path indicated by Ie of FIG. 10(b) flows in the pixel at the time when a row is not selected. A potential of the node 105 is retained between period of (a) and (b) by a capacitor 65. Therefore, if characteristics of transistors 82a and 82b are equal, Iw=Ie.
It is likely that a current flowing to an organic luminous element changes with respect to fluctuation in transistor characteristics. However, since the transistors 82a and 82b are located in the identical pixel circuit and arranged close to each other compared with the case of the voltage drive, it is possible to reduce fluctuation in a current. Unevenness of display is small compared with the voltage drive.
Therefore, it is necessary to use the current drive system in order to obtain uniform display. For that purpose, the source driver 17 must be a driver IC of a current output type.
An example of an output stage of a current driver IC, which outputs a value of current according to a gradation, is shown in FIG. 11. An analog current is outputted to display gradation data 115 from 114 by a digital/analog conversion unit 116. The analog/digital conversion unit is constituted by plural (at least the number of bits of the gradation data 115) current sources for gradation display 113 and switches 118, and a common gate line 117 which regulates a value of current flown by one current source for gradation display 113.
In FIG. 11, an analog current is outputted in response to the input 115 of three bits. It is selected by the switches 118 whether the current sources 113 of the number corresponding to a weight of bits are connected to the current output 114, whereby a current corresponding to a gradation can be outputted in such a manner that a current equivalent to one current source 113 is outputted in the case of data 1 and a current equivalent to seven current sources 113 is outputted in the case of data 7. A current output type driver can be realized by arranging 116 of this structure by the number corresponding to the number of outputs of the driver. In order to compensate for a temperature characteristic of the transistors 113, a voltage of the common gate line 117 is determined by a distributing mirror transistor 112. The transistor 112 and the current source group 113 are formed in a current mirror structure, and a current per one gradation is determined according to a value of a reference current 19. With this structure, an output current changes according to a gradation, and a current per one gradation is determined according to a reference current.
Examples of a display device using an organic luminous element are shown in FIGS. 12 to 14. FIGS. 12(a) and (b) show a television, FIG. 13 shows a digital camera or a digital video camera, and FIG. 14 shows a personal digital assistant. Since a response speed of the organic luminous element is high, the organic luminance element is a display panel suitable for these display devices which has many opportunities to display motion images.
When the number of source signal lines of these display devices increases, as shown in FIG. 15, it is required to use plural current output type source driver ICs 17 with respect to one display panel 151. In this case, if output currents of the driver ICs 17 have fluctuation of 1% or more among chips, luminance is different in each display area of 152a and 152c, and block unevenness is caused. Therefore, a measure for cascade connection of the driver ICs 17 is required.
In order to reduce fluctuation of output currents among different chips, it is necessary to make a value of the reference current 19 uniform. As a conventional technique of making a reference current uniform, there are known systems such as shown in FIG. 16 (e.g., see Japanese Patent Application Laid-Open No. 2000-293245) and FIG. 19. Note that, since there is no detailed description about an output stage 164, the output stage shown in FIG. 3 is applied.
In the method of FIG. 16, one original current 161 is inputted to reference current distribution units 162 in the driver ICs 17 to generate plural reference currents. A circuit structure example of a reference current distribution unit is shown in FIG. 17. Plural reference currents 163a to 163c are outputted to the original current 161 by a current mirror circuit. These reference currents 163 are supplied to the driver ICs 17, respectively, to make reference currents of all the driver ICs 17, which are connected in cascade, equal, whereby the block unevenness is eliminated. Note that there is also a method in which 161 is voltage input rather than a current, and the voltage input is connected to a connection line 185 to an operational amplifier 183 shown in FIG. 18. The voltage is changed to a current by a current/voltage conversion unit 184, and a current flowing to a resistor 182 becomes an original current. There is also a method in which this current is distributed by a current mirror to output the reference current 163. The resistor 182 may be incorporated in the driver ICs 17 or may be provided externally.
In a second method of supplying an identical reference current to the plural driver ICs 17 shown in FIG. 19, the reference current 161 is directly inputted to an output stage 164 in a first driver 17a. In a current delivery unit 191, a transistor 201 is connected to a common data line 117 to take a current mirror structure with respect to a reference current. If the reference current is outputted directly, a direction of the current is reversed. Thus, the current direction is further changed by a current mirror structure consisting of transistors 202 and 203, whereby the reference current is supplied to another driver IC 17 by 192. Consequently, the identical reference current flows to the respective driver ICs. Note that the entire disclosure of Japanese Patent Application Laid-Open No. 2000-293245 described above is incorporated herein by reference in its entirety.
In the structure of FIG. 16, in the case in which a display device using plural driver ICs is manufactured, the number of driver ICs, which can be arranged in plural form, is limited by the number of current outputs of the reference current distribution unit 162. For example, in the structure of FIG. 17, only three driver ICs can be arranged at the maximum. In addition, if 162 is arranged in the outside, since another semiconductor circuit is required, there is a problem in that packaging cost and a module structure are complicated.
In addition, in a structure shown in FIG. 20, it is necessary to pass a current through a current mirror circuit twice in order to generate a reference current, and fluctuation tends to increase. Since it is necessary to increase a transistor size in order to control the fluctuation, it is required to increase a chip size. In addition, a current passes through a current mirror of two stages for one chip, in the case of N (N: natural number) chip connection, since the current mirror is repeated 2N times, the original current 161 and an Nth reference current tend to deviate from each other.
In addition, in a method of regulating a current per one gradation of each output with a current mirror using the structure of FIG. 11 in a structure of an output stage as well, as a transistor of a mirror source and a transistor for output are further apart from each other, the current is affected by fluctuation of characteristics of the transistors more easily due to characteristics of the current mirror.
For example, the distributing mirror transistor 112 is arranged at a left end, and the digital analog conversion unit 116 is arranged in the vicinity of an output pad. In that case, if there is shift of a threshold voltage as shown in FIG. 21(a) in a silicon wafer, output currents are different at left and right ends of the driver IC as shown in FIG. 21(b). For example, when the threshold voltage fluctuates as indicated by 211a, since a voltage of the common gate line 117 forming the current mirror is uniform, the current output at the right end decreases compared with the mirror source at the left end. Similarly, relations as indicated by 212ab and 213ab are obtained.
Therefore, even if a reference current value is uniform in all the drivers IC 17, in the case in which plural driver ICs having such an output characteristic are arranged, a difference is caused in current values at boundaries of the driver ICs as shown in FIG. 22. Consequently, block unevenness is caused.
In order to eliminate unevenness for each IC in current driver ICs, there are objects of making it possible to input a reference current uniformly and making current values at left and right ends of an identical chip the same.